The invention relates to a method of manufacturing an optical fiber preform, the method comprising forming at least one silica-based external deposition layer by depositing silica on a primary preform consisting in a bar mainly comprising silica and including a silica-based outer peripheral portion.
In known manner, external deposition is performed on a primary preform, e.g. as shown in FIG. 1 which relates to plasma build-up, in order to enlarge the diameter of the preform, thereby increasing the length of the optical fiber, as shown in FIG. 2, which can be made by hot drawing the resulting preform. The primary preform, whether it is made by the modified chemical vapor deposition (MCVD) technique or by the vapor axial deposition (VAD) technique, is a bar mainly comprising silica and including an outer peripheral portion which is based on silica, said outer peripheral portion usually consisting in a silica-based tube forming the side wall of the primary preform, with said bar comprising, inside the outer peripheral portion, optical cladding and a core which have different refractive indices. Silica, generally in the form of a silica grains when the external deposition is performed by plasma build-up, is deposited on the silica-based outer peripheral portion of the primary preform so that the build-up layer has substantially the same refractive index as said peripheral portion.
FIG. 1 shows, in highly diagrammatic manner, plasma build-up apparatus comprising an enclosure 1 having a transparent window 2, a preform 3 of longitudinal axis X seen end-on, and onto which there are directed a plasma torch 4 and a nozzle 5 for supplying build-up grains. Outside the enclosure 1, a CCD camera 6 located behind the window 2 is directed towards the preform 3. It provides a measurement of the diameter of the preform at the location towards which it is pointed and in the form of a value which is transmitted over a link 7 to apparatus 8 for controlling the build-up process. Over a multiple link 9, the apparatus 8 also receives other indications about build-up process conditions. Under the effect of an internal program for controlling the build-up process, and at constant grain feed rate, the apparatus 8 delivers over an outlet link 10 connected to a control apparatus 11, a control value for positioning the nozzle 5 relative to the preform 3 so that the nozzle 5 is positioned accordingly by being displaced along an axis parallel to the longitudinal X of the preform 3. Over a multiple output link 12, the apparatus 8 also delivers other control values that govern other aspects of the control process.
All of the elements of the apparatus shown in FIG. 1 are well known to the person skilled in the art. Other elements (not shown) are also well known. This applies to means for supporting the preform 3 while enabling it to be driven in rotation and in translation, a carriage for supporting the plasma torch 4 and the nozzle 5, and suitable for being driven in translation parallel to the longitudinal axis of the preform 3, and means for evaluating the angular position of the preform 3 and the longitudinal position of the carriage, e.g. as described in European patent application EP-A1-0 440 130. In conventional manner, these means together enable the preform 3 to be moved away from the torch 4 as the preform 3 becomes larger. Means for directing the camera 6 to successive locations on the preform 3 during a measurement pass likewise form part of the prior art and can be constituted by a second carriage whose displacement is coupled to that of the first carriage.
Plasma build-up takes place in passes, from right to left and then from left to right, during which the plasma torch 4 and the nozzle 5 scan the length of the preform 3.
The entire control process is optimized so as to obtain high yield concerning the quantity of silica deposited at a given speed of translation and for a given refractive index for the build-up layer.
Simultaneously, the camera 6 performs a measurement pass, providing successive values for the diameter of the preform 3 along its entire length. In order to simplify matters, it is preferable for the camera 6 to perform measurement only on every other pass, e.g. on the right to left pass, while during the following pass the output from the camera 6 is not enabled.
FIG. 2 is a diagrammatic section view through an optical fiber 15 made by hot drawing down of a preform 3 obtained by a silica-based external deposition method applied to a primary preform 24, e.g. a preform made by the MCVD method. The layers of the optical fiber 15 and of the preform 3 correspond, ignoring scale, so the same numerals 20 to 24 are used both for the preform 3 and for the optical fiber 15 shown in FIG. 2. The primary preform 24 comprises an optical core 20, cladding 21, and an outer peripheral portion 22. The primary preform 24 is made using the MCVD method by internal deposition of optionally-doped silica-based layers forming the optical core 20 and the optical cladding 21 inside the tube 22, followed by the tube that has been internally coated in this way being transformed into a bar by being collapsed, which bar constitutes the primary preform 24, after which the final preform 3 is made by external silica-based deposition of external deposition layers 23 deposited on the primary preform 24.
The problem which arises is that of obtaining an optical fiber having a refractive index that is constant throughout its thicknesses which is scaled, during the hot drawing operation, from both the silica-based outer deposition layer and the silica-based outer peripheral portion of the primary preform. It has been observed that an undesirable index step is to be found in the optical fiber that results from hot drawing of the preform, said step being located (ignoring scale) at the boundary between the outer peripheral portion and the silica-based outer deposition layer, said outer peripheral portion apparently not being subjected to the same compression stresses during hot drawing as is the outer deposition layer.
To this end, the invention provides a method of manufacturing an optical fiber preform comprising: forming at least one silica-based outer deposition layer by depositing silica on a primary preform constituted by a bar mainly comprising silica and including a silica-based outer peripheral portion, the method being characterized in that the viscosity of the outer deposition layer is adjusted to be substantially identical to the viscosity of the outer peripheral portion of the primary preform by adding to the silica, over a substantial portion of the outer deposition layer, at least one compound selected from the group formed by the following compounds: CaF2, MgF2, AlF3, B2O3, and Al2O3.
Preferably, if the dopant is selected from the group constituted by CaF2, MgF2, and AlF3, the proportion of the dopant lies in the range 5 ppm to 100 ppm, preferably in the range 10 ppm to 40 ppm, by weight of the fluorided element (i.e. the calcium, the magnesium, or the aluminum) relative to silica. It is necessary to have sufficient dopant in order to obtain a significant effect on viscosity. Furthermore, if the proportion of said dopant is excessive, a problem is observed concerning the stability of the diameter of the optical fiber manufactured from the preform, and also variation of the index around the nominal value can be too great compared with the tolerance range set by the manufacturer.
Preferably, if the dopant is selected from the group formed by B2O3 and Al2O3, the proportion of dopant lies in the range 0.01% to 1%, preferably in the range 0.1% to 0.6%, by weight of the oxidized element (i.e. the boron or the aluminum) relative to the silica. It is necessary to have sufficient dopant to obtain a significant effect on viscosity. Furthermore, if the proportion of said dopant is excessive, there is observed, on the contrary, a new index step because the dopant has softened the silica excessively.
In an implementation of the invention, the silica is in the form of grains of a size generally lying in the range 50 xcexcm to 300 xcexcm.
By doping the silica with a dopant in the above-described preferred proportions, an external deposition layer is formed which possesses viscosity during hot drawing that is substantially equal to that of the outer peripheral portion of the primary preform, thus leading to drawing which is uniform in a plane extending transversely to the preform, the silica-based outer peripheral portion and the outer deposition layer being in substantially the same stress state. As a result, the thicknesses of optical fiber scaled from the build-up layer and from the silica-based outer peripheral portion retain the same refractive index.
Advantageously, the proportion of dopant(s) used is such that dopant diffusion through the silica-based outer peripheral portion in the cladding and in the core of the optical fiber is not significant in terms of having a prejudicial effect on optical attenuation properties.
The dopant is preferably supplied in a proportion relative to that of silica that enables the viscosity of the build-up layer and of the outer peripheral portion of the primary preform to be adjusted without significantly modifying the refractive index of the build-up layer relative to that of said silica-based outer peripheral portion.